1. Field of the Invention
The present invention relates to a constant current circuit to be formed on a chip of a semiconductor integrated circuit, and more particularly, to a constant current circuit including start-up means for preventing oscillation when power is input.
2. Description of the Related Art
Constant current circuits are used as current sources for circuits in various types of electronic devices. It is a function of the constant current circuit to output a constant current to an output terminal independently of power supply fluctuations at a power supply terminal. Achieving lower current consumption operation is also an important issue for the constant current circuit.
FIG. 4 illustrates a circuit diagram of a conventional constant current circuit. The conventional constant current circuit includes a constant current circuit section 410 and a determination circuit section 411. The constant current circuit section 410 has an output connected to a gate of a P-channel transistor 407 included in the determination circuit section 411. The determination circuit section 411 has an output connected to a gate of an N-channel transistor 406 included in the constant current circuit section 410.
Next, an operation of the conventional constant current circuit is described.
Immediately after power is input, a potential of an output terminal 422 of the constant current circuit section 410 is still zero, but increases as a power supply voltage 130 increases. When a difference between the voltage of the output terminal 422 and the power supply voltage 130 becomes lower than a threshold voltage of the P-channel transistor 407, the P-channel transistor 407 enters an OFF state. At this time, a potential of a node C is zero and hence a potential of an output terminal of an inverter 408 is High. Accordingly, the N-channel transistor 406 enters an ON state and the potential of the output terminal 422 becomes zero. Then, each gate potential of a P-channel transistor 401 and a P-channel transistor 402 included in the constant current circuit section 410 becomes zero, and hence currents I1 and I2 are excited to nodes A and B, respectively (hereinafter, this operation is referred to as current exciting operation). At the same time as the current excitation, a gate potential of the P-channel transistor 407 decreases so that a current flows through the node C and a load resistor 409. If design is made such that the potential of the node C on this occasion exceeds a logic threshold of the inverter 408, the potential of the output terminal of the inverter 408 may be inverted to zero so that the N-channel transistor 406 enters an OFF state.
In the event that the constant current circuit section 410 cannot be enabled by the excitation currents I1 and I2, a potential of the node B increases to turn OFF the P-channel transistor 407 eventually. Then, the determination circuit section 411 is shifted to the above-mentioned current exciting operation to excite the currents I1 and I2 again to the constant current circuit section 410.
In such a way, the determination circuit section 411 excites the currents I1 and I2 as many times as needed until the constant current circuit section 410 is enabled, to thereby reliably start up the constant current circuit and make a shift to a “constant current state” (see, for example, Japanese Patent Application Laid-open No. Hei 07-106869).
The description above is given to an example where the resistor 409 is used in the determination circuit section 411 as means for converting ON/OFF of the P-channel transistor 407 into a start-up signal. However, the resistor 409 may be replaced with a depletion type N-channel transistor. Specifically, a drain electrode of the depletion type N-channel transistor is connected to the node C of the determination circuit section 411, and gate and source electrodes thereof are connected in common to a ground potential 131. With this connection, the depletion type N-channel transistor may operate as one whose gate-bias voltage is always zero. This provides, as already well known, the effect of reducing an area of a resistor in a circuit requiring high resistance.
However, in the conventional technology, while the start-up state of the constant current circuit section 410 is monitored based on the node B, the excitation current for start-up is supplied to the node B. If the supply of the excitation current is ended before the node A of the constant current circuit section 410 is shifted to the start-up state, the constant current circuit is not allowed to start up and returns into a zero steady state again. This leads to a fear that the constant current circuit repeats the start-up state and the zero steady state to enter an oscillating state. Further, after the start-up of the constant current circuit, a current flows through the determination circuit section 411 all the time, which is not suitable for lower current consumption.